The spread of Dreissena polymorpha, the zebra mussel, to North America presents a classic example of how a foreign species of an organism can spread, unchecked by natural predators, to create major problems. Dreissena polymorpha, a sessile fresh water mollusk native to the Caspian Sea in Russia, is thought to have been transported to North American waters in about 1986 in the ballast of a ship. After its discharge into Lake St. Clair, the zebra mussel has multiplied rapidly and spread into all of the Great Lakes and many contiguous waterways as well, encrusting the hulls and engines of boats and the components of waterworks, cooling systems and other water distribution system structures in these waters. The tendency of this tenacious mollusk to form large colonies has created untold damage. There is currently no safe, effective and reliable way to prevent, or even control, the unwanted spread and biofouling produced by the zebra mussel.
A female zebra mussel produces about 30,000 eggs annually. When the eggs have been fertilized, they hatch into microscopic larvae which are carried randomly by currents until they mature sufficiently to attach to whatever hard surfaces they happen to contact. Once a zebra mussel larva fastens itself to a hard surface, the maturation process is completed, and each mature mussel has formed a hard hinged shell. The mussel then remains sessile in this location for the rest of its life. Zebra mussels will attach to virtually any hard underwater surface and are especially adherent to the shells of other zebra mussels. Consequently, large clumps of zebra mussels are quickly formed on surfaces where only one or a small number of larvae had originally attached.
The zebra mussel, which has a characteristic arrangement of dark and light brown stripes on its shell, attaches itself to firm objects by thread-like tentacles. These tentacles are tough fibers of a horny material and are known as byssal threads or byssuses. The byssal threads secrete an adhesive which enables the mussel to affix itself to any surface, no matter what its orientation relative to the horizontal. As a result, zebra mussels have attached themselves to pipes, valving, cooling equipment, intake and outflow lines and other components of such water distribution systems as municipal water treatment systems, industrial water systems, power plant cooling systems, and even those of marine vessels. Colonies of zebra mussels which can quickly reach a population density of 30,000 mussels per square meter, present substantial and significant problems to these systems.
The growth of zebra mussel colonies in pipes and other conduits in contact with fresh water sources reduces the effective bore of the conduits and increases the roughness factor, thus diminishing their water carrying capacity. In addition, by creating local differences in the oxidation state on the inner surface of cast iron and steel pipes, zebra mussel colonies can promote electro-corrosion of these pipes. At least one municipality whose water supply comes from one of the Great Lakes has had its municipal water supply cut off because the intake lines were clogged with zebra mussel colonies. Industrial plants have had the flow of essential cooling water interrupted by zebra mussel infestations.
Zebra mussel colonies threaten virtually every municipality, industry and utility that draws water from the rivers, lakes and streams where these organisms have spread. Because the motile zebra mussel larvae forms range from microscopic to very tiny in size they are easily drawn through water intake screens and other components and deep inside water treatment or cooling systems. Once inside these systems, the immature mussels can lodge anywhere, and form bio-fouling colonies.
Once the magnitude of the zebra mussel population explosion in North America was fully appreciated, efforts to prevent and control this pest were proposed. Because Dreissena polymorpha has no natural predators in these waters, eradication of the mussels has focused primarily on manually scraping the colonies from intake pipes and other equipment. However, this method is slow, expensive and does not guarantee the removal of every mussel. Additionally, the equipment being scraped cannot be used and must be out of service until mussel removal has been completed. Although many European water distribution systems employ dual water intake lines so one can be used while the other is shut down for zebra mussel removal, this is not an inexpensive modification for an existing water distribution system.
Because the zebra mussel veliger larvae forms do not survive at temperatures below about 13.degree. C., it has been suggested that water intake and other equipment be located at water depths where the temperature is not likely to rise above 13.degree. C. However, this is not always possible. Even where it is feasible, it could be quite costly to relocate existing system components.
The application of up to 50 ppm chlorine to water where zebra mussel removal is required for a period of about two weeks has been proposed, and appears to be a reliable control method. However, the potential toxicity of the chlorine concentrations required to eradicate mussels to other animals and humans and the undesirable chlorinated organic compounds formed by chlorine are major disadvantages to this method. Moreover, chlorine can be quite corrosive to water distribution system operating components.
Other chemical methods have been proposed to combat zebra mussels and harmful marine mollusks. For example, U.S. Pat. No. 4,154,818 to Kanada et al. discloses a gel product containing a molluscicide for exterminating such harmful marine mollusks as Teredo, Mytilis edulis, barnacles and Hydrozoa which tend to adher to vessel walls and other surfaces. The gel product is suspended in a mesh container or the like in the water near the infestation, and the molluscicide gradually dissolves. Although this method should eradicate an infestation of mollusks in the vicinity of the gel, the concentration of the molluscicide in the vicinity of the gel is not subject to precise control, but is dependent on water conditions and could be high enough to be toxic to desirable organisms as well as the mollusks to be exterminated.
U.S. Pat. No. 5,015,395 to Muia et al. discloses a method for controlling zebra mussels in aqueous systems by adding to the system a water soluble dialkyl diallyl quaternary ammonium polymer. This compound, while apparently effective in killing zebra mussels, also effectively kills other aquatic life if the concentration is not precisely controlled.
U.S. Pat. No. 5,040,487 to Bollyky et al. discloses a method of controlling zebra mussel infestations in waterworks by introducing ozone into the water entering the system as close to the crib of the water intake as possible. Additional applications are required if the water line is long or the system is complex. This method requires the relatively precise application of ozone from a separate ozone source to be effective. Additionally, it is most effective against eggs and larvae forms, and will only kill immature mussels before they have attached to a surface.
It has also been proposed to selectively eradicate zebra mussels with potassium by adding potassium to paint applied to underwater structures since potassium is apparently toxic to zebra mussels. While this may be promising, its efficacy has yet to be demonstrated.
The prior art, therefore has failed to provide a safe, reliable method or apparatus for preventing the damage caused by zebra mussel or other undesirable sessile mollusk infestations to water distribution systems with components which contact bodies of fresh water infested with zebra mussels or undesirable sessile mollusks that does not require the application to the water of a chemical that is potentially toxic to other aquatic life.